Glycosyltransferases are enzymes involved in in vivo biosynthesis of sugar chains on glycoproteins, glycolipids and the like. Their reaction products, i.e., sugar chains on glycoproteins, glycolipids and the like (hereinafter referred to as “complex carbohydrate sugar chains”) have very important functions in the body. For example, sugar chains have been shown to be important molecules primarily in mammalian cells, which play a role in cell-cell and cell-extracellular matrix signaling and serve as tags for complex carbohydrates during differentiation and/or development.
As described above, sugar chains have very important functions. Erythropoietin can be presented as a specific example where sugar chains are applied. Although erythropoietin is inherently a glycoprotein, further attempts have been made to increase the number of sugar chains on erythropoietin, as a result of which recombinant erythropoietin proteins with an extended life span have been produced and are now commercially available.
It can also be expected that in the future, further developments will take place for such products in which sugar chains are applied, including pharmaceuticals and functional foods. For this reason, glycosyltransferases will increase in importance as a means for synthesizing and producing sugar chains.
Until now, about 150 or more glycosyltransferase genes have been isolated from eukaryotic organisms including humans, mice, rats and yeast, and proteins having glycosyltransferase activity have also been expressed in production systems where CHO or E. coli cells are used as host cells. On the other hand, several glycosyltransferase genes have also been isolated from bacteria which are prokaryotic organisms. Moreover, proteins having glycosyltransferase activity have been expressed in recombinant production systems using E. coli and identified for their substrate specificity and/or various enzymatic properties.
Among sugars constituting sugar chains, sialic acid that is often located at the nonreducing termini is a very important sugar in terms of sugar chain functions, and hence sialyltransferase is one of the most in demand enzymes among glycosyltransferases, which are now increasing in importance.
As to α2,3-sialyltransferases and their genes, many reports have been issued for those derived from animals, particularly mammals (see, e.g., Harduin-Lepers, A. et al., Biochem J., 15; 352 Pt 1:37-48 (2000); Young-Choon Lee et al., J. Biol. Chem., 23; 274(17):11958-67 (1999); Lee, Y-C. et al., J. Biochem., 216, 377-385 (19.93); Chang, M-L. et al., Glycobiology, 5, 319-325 (1995); and Gillespie, W. et al., J. Biol. Chem., 267, 21004-21010 (1992)). However, such animal-derived enzymes are very expensive because they are difficult to purify and hence cannot be obtained in large amounts. Moreover, such enzymes have a problem in that they have poor stability as enzymes.
In contrast, as microorganism-derived α2,3-sialyltransferases and their genes, those obtained from microorganisms belonging to the genera Neisseria, Campylobacter, Haemophilus and Pasteurella have been reported (see, e.g., WO97/047749, WO99/049051, WO01/077314, WO03/027297). However, there is no report showing that α2,3-sialyltransferase has been obtained from microorganisms belonging to the Vibrionaceae, and there is also no report showing that a protein having α2,3-sialyltransferase activity is present in microorganisms belonging to the Vibrionaceae.    Patent Document 1: International Publication No. WO97/047749A    Patent Document 2: International Publication No. WO99/049051A    Patent Document 3: International Publication No. WO01/077314A    Patent Document 4: International Publication No. WO03/027297A    Non-patent Document 1: Harduin-Lepers, A. et al., Biochem. J., 15; 352 Pt 1:37-48 (2000)    Non-patent Document 2: Young-Choon Lee at al., J. Biol. Chem., 23; 274(17):11958-67 (1999)    Non-patent Document 3: Lee, Y-C. et al., J. Biochem., 216, 377-385 (1993)    Non-patent Document 4: Chang, M-L. et al., Glycobiology, 5, 319-325 (1995)    Non-patent Document 5: Gillespie, W. et al., J. Biol. Chem., 267, 21004-21010 (1992)